The present invention relates to methods and apparatus for obtaining counter-current gas-liquid contact, such as wet scrubbers which involve gas-liquid contact between flue gas containing sulfur oxides and a slurry or solution containing reactive materials as a chemical reacting absorbing medium. More particularly, the present invention is drawn to an improved wet scrubber tray construction and a gas-liquid contact apparatus employing same.
Wet scrubber gas-liquid contact systems used for the removal of sulfur oxides (SOx) from flue gases produced from the combustion of fossil fuels are well known. See for example U.S. Pat. No. 4,263,021 to Downs et al., the text of which is hereby incorporated by reference as though fully set forth herein. Additional details of wet scrubber systems for SOx removal are provided in Chapter 35 of Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein. Such systems are also referred to as wet flue gas desulfurization (WFGD) systems.
FIGS. 1 and 2 illustrate components of one type of known wet scrubber system, generally designated 10, described in the aforementioned Steam 41st reference. SO2-laden flue gas 12 enters the side of the spray tower absorber 14 at approximately its midpoint and exits through a transition 16 at the top. The upper portion of the module (absorption zone 18) provides for the scrubbing of the flue gas 12 to remove the SO2 while the lower portion of the module serves as an integral slurry reaction tank (also frequently referred to as the recirculation tank and oxidation zone 20) to complete the chemical reactions to produce gypsum. Other key components shown include the slurry recirculation pumps 22, spray headers 24 (which may be conventional or interspatial design) and nozzles for slurry injection, moisture separators 26 (typically of a chevron design) to minimize moisture carryover, oxidizing air injection system 28, slurry reaction tank mixers or agitators 30 to prevent settling, and a perforated tray 32 that reduces flue gas flow maldistribution to enhance SO2 removal performance. Flues leading from the transition 16 convey the cleaned flue gas 34 from the wet scrubber 14.
The perforated tray 32 is provided with a plurality of holes 36 (typically 1⅜ inches diameter) and provides intimate gas/liquid contacting and increases the slurry residence time in the absorption zone. The tray 32 creates more surface area between the slurry and the flue gas 12, and also provides significant holdup time for the slurry. This increases the limestone dissolution in the absorption zone 18 and increases the absorption per unit volume. Some spray tower absorbers 14 have two levels of trays providing multiple contact zones for SO2 removal. Absorber modules that do not use a tray are referred to as open spray towers.
Recently, after a period of operation, some perforated trays have begun to exhibit cracking. This cracking has appeared in two areas.
Crack site 1. In the center stiffener, cracks were found at the end of the stiffener adjacent to a field weld joining the affected tray with its neighboring tray. These cracks were found to be initiating from the field welds, traveling horizontal into the center stiffener web. The orientation of the crack suggested that it was following a line of maximum bending stress caused by lateral movement of the top corner of the center stiffener. The forces causing this movement were thought to be random buffeting of the stiffener by flue gas and/or slurry spray. The initiation point at the field weld was caused by the Fatigue Strength Reduction Factor (FSRF) inherent to all welds.
Crack site 2. In the perforated bottom plate region adjacent to both the intermittent shop welds joining the center stiffener to the perforated bottom plate, and in the intermittent field welds connecting the trays to the steel support grid. The field welds are located at the end edges of the perforated bottom plate. The cracks were observed to initiate at the ends of the weld segments, traveling to the nearest hole in the perforated bottom plate. There were some instances where the cracks would initiate at both ends of a series of weld segments. This condition would effect the support boundary conditions of the perforated bottom plate in such a way that chaining of cracks would occur. This chaining effect had the potential of compromising the structural integrity of the perforated bottom plate, causing large sections of the perforated bottom plate to fall out, reducing the effectiveness of the function of the tray and creating a risk of pieces being sucked into the slurry recirculation pumps.
Metallurgical examinations confirmed that the cracking was due to mechanical fatigue. This fatigue was thought to be induced by mechanical vibration from an unknown origin. No evidence of stress corrosion cracking or brittle fracture was found. The heat affected zone (HAZ) adjacent to the welds was also examined and found to be in satisfactory condition.